Is the promise of waste based aviation fuel sending us down a blind alley?

This post is by Cait Hewitt, deputy director of the Aviation Environment Federation.

Scientists in America have found a way to massively reduce emissions from flying by using a new fuel made from waste, a BBC news headlines announced on 15 March. It sounded like the kind of scientific breakthrough that almost everyone would want to see: tackling waste and reducing emissions while allowing people to carry on flying. In fact, the story went on to report, the new fuel can reduce greenhouse gas emissions by 165 per cent suggesting that one way to lower emissions would be to fly more.

It smelled fishy, and so it turned out to be. The study, published by an American research centre, showed that it was possible to turn wet waste and animal manure into aviation fuel through a process of catalytic conversion. That all seems ok. But there are two problems with the way this finding was reported: first, the suggestion that this will generate a big reduction in airline emissions (in fact tailpipe emissions will be about the same as burning kerosene); and, second, the approach to carbon accounting, which masks the fact that there is no true carbon removal associated with this fuel.

This can’t magic away emissions
The claimed emissions saving, reported the BBC, “comes from the reduction in carbon emitted from airplanes plus the emissions that are avoided when food waste is diverted from landfill.”  In fact, the carbon released when the fuel is burned in an aircraft, would – an annex to the study notes – be slightly higher than that from fossil fuel. The emissions saving is derived from an estimate, partly, of the avoided methane emissions associated with landfill waste, and partly from an assumption that using a ‘biogenic’ source of carbon means that the emissions from burning it can be assumed to have been offset.

Tackling the methane coming from waste will be important to delivering climate goals, and it seems to make sense to use energy from materials that have already been extracted. The problem is that, to reach net zero emissions, we will need to take steps like this as well as – not as a replacement for – achieving net zero aviation. The end result of burning this fuel will not be a net reduction in emissions, it will just mean that some emissions that might have arisen from landfill, or from extracting raw materials, will be avoided.

Sustainable aviation fuels are back on the agenda
The aviation industry in the UK has been lobbying hard for government investment in ‘sustainable aviation fuels’ or SAFs. These are ‘drop-in’ fuels which, unlike hydrogen or electricity, don’t require new aircraft designs. Airlines know that they can’t escape scrutiny much longer for their climate impact, especially as emissions from flights departing the UK were higher in 2019 (just before the pandemic) than ever before. And, while some airlines are still relying on carbon offsetting to bail them out, there’s widespread public scepticism about this. Many airlines now acknowledge that it can’t be a long term solution.

In many ways, though, SAFs are themselves a form of offset, either avoiding or removing emissions from elsewhere to help balance the CO2 coming out of the plane’s exhaust pipe. The concept behind crop-based biofuels was that the plants would first absorb the CO2 before it was then released back into the atmosphere via an aircraft engine. But social justice and environmental concerns quickly emerged, related to competition with agriculture and with forestry, and fears that monocultural plantations would be bad for biodiversity.

As a result, there’s a been a shift towards producing fuel from various forms of wastes (municipal solid waste, plant and animal fats, and other agricultural wastes for example), supported in the UK by small financial incentives through the Renewable Fuels Transport Obligation. And yet, even if waste based SAF could be made cost competitive with jet fuel (it’s currently at least twice as expensive), it would seem necessarily to be limited in supply.

There are alternatives to waste based SAFs. In particular, Climate Change Committee (CCC) modelling assumes some uptake of synthetic aviation fuel, produced using direct air capture of carbon, combined with hydrogen, using processes powered by renewable electricity. This could represent a net zero carbon option for aviation but would require a huge scale-up of renewable energy, and of direct air capture technology, both of which currently face significant cost and other barriers.

One potential benefit of SAFs is that some types could generate less soot on burning, reducing the likelihood of contrail formation, which has an additional warming impact. But this doesn’t remove the fundamental issues in relation to CO2, which has easily the most long lasting climate impact.

Zero emission flying is still a long way off
As a report for the CCC put it, flying is “the quickest and cheapest way for a consumer to increase their carbon footprint.” With just thirty years to go before aviation, like all other sectors, will have to achieve net zero emissions, and with the industry increasingly under the spotlight when it comes to climate change, drop-in decarbonisation options that don’t require changes to aircraft or to airport structures have obvious appeal.

But pursuing aviation fuels whose primary advantage is to finance the avoidance of landfill emissions could send us down a blind alley in the search for answers to the aviation challenge. For flying to be possible in a net zero future, we will need either to power our aircraft in a way that produces no emissions, or capture carbon, whether to make synthetic fuel or by deploying technologies to remove and permanently store carbon to balance out total atmospheric CO2.

We are a very long way from being ready to roll out these options, and airlines currently have no meaningful climate obligations or financial incentives to make sure they deliver solutions. We shouldn’t be wasting time on developing fuels that can’t, in the long run, deliver net zero.

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